1. Technical Field
Embodiments of the present disclosure generally relate to photomasks used in fabrication of semiconductor devices and methods of fabricating the photomasks and, more particularly, to reflection-type photomasks used in fabrication of semiconductor devices and methods of fabricating the reflection-type photomasks.
2. Related Art
Integration density of semiconductor devices has been continuously increased with the development of a nano-technology. As the semiconductor devices become more highly integrated, some elements such as transistors and interconnection lines have been scaled down to reduce sizes thereof. Next-generation semiconductor technologies may depend on process technologies, which provide the possibility of formation of fine patterns, rather than circuit design technologies. The formation of the fine patterns may be closely related with photolithography process technologies. The photolithography process may include forming a photoresist layer on a semiconductor substrate, transferring desired pattern images onto the photoresist layer using a light to selectively expose portions of the photoresist layer, and developing the exposed photoresist layer with a developer to form photoresist patterns. The light used in the exposure step may directly affect a resolution of the photoresist patterns.
The resolution of the photoresist patterns may depend on a wavelength of the light used in the exposure step of the photolithography process. Various lights may be used in the exposure step of the photolithography process. For example, the light used in the exposure step of the photolithography process may be one of an i-line beam having a wavelength of about 365 nanometers, a KrF laser beam having a wavelength of about 248 nanometers and an ArF laser beam having a wavelength of about 193 nanometers. Even though the lights having short wavelengths such as the i-line beam, the KrF laser beam and the ArF laser beam are used in the exposure step of the photolithography process, there may be still limitations in enhancing the pattern resolutions of the highly integrated semiconductor devices. Thus, in recent years, extreme ultraviolet (EUV) rays referred to as soft X-rays have been revealed to overcome the limitations of the photolithography processes utilizing the i-line beam, the KrF laser beam or the ArF laser beam as a light source.
The EUV ray has a wavelength of about 13.5 nanometers which is considerably less than those of the i-line beam, the KrF laser beam or the ArF laser beam. Since the wavelength of the light used in the exposure step directly affect the pattern resolution, the EUV lithography process utilizing the EUV ray as a light source may exhibit a remarkably high pattern resolution as compared with the photolithography process utilizing the i-line beam, the KrF laser beam or the ArF laser beam as a light source. However, the EUV ray may have high energy because the wavelength of the EUV ray is too short. Thus, it may be difficult to use the typical transmission-type photomasks as the photomasks of the EUV lithography process. This is because transparent substrates of the typical transmission-type photomasks are too weak to endure the high energy of the EUV ray and the energy efficiency of the EUV ray is too low to enhance the pattern resolution. Accordingly, reflection-type photomasks have been proposed to overcome the disadvantages of the typical transmission-type photomasks. When the reflection-type photomasks are used in the EUV lithography process, the EUV ray may be reflected on surfaces of the reflection-type photomasks without penetrating the reflection-type photomasks. Hence, the high energy of the EUV ray may not be applied to the substrates of the reflection-type photomasks.